Bombesin-related peptides ([gastrin-releasing peptide [GRP], neuromedin B) interact with two distinct receptors (GRP-R, NMB-R) to mediate a number of effects in the gastrointestinal tract (GI), central nervous sytem (CNS) and on growth of normal and neoplastic tissues. Furthermore, two related receptors, a mammalian orphan receptor (BRS-3), having 60% homology to GRP-R and a novel receptor in amphibians, BB-4-R has been described recently. The aims of this project are to understand the pharmacology, molecular pharmacology, and cell biology of these receptors as well as to develop specific agonists and antagonists that can be used to determine their physiological roles. Investigations being performed include expression of these receptors in stable cell lines that resemble native receptors in their cell biology and pharmacology; investigations using site-directed mutagenesis and receptor chimeras to define receptor structural determinants of ligand selectivity and specificity for agonists and antagonists, pharmacological studies of BN-related peptides to identify selective agonists/antagonists and studies of native cells and transfected cells to define the transduction cascades of these receptors. Both the orphan receptor, BRS-3 and the newly described BB-4-R receptors have been stably expressed allowing detailed pharmacology and cell biology to be performed, because no cell lines expressing sufficient native receptors exist. A high affinity ligand for the BRS-3 receptor [DPhe-6,Beta Ala,Phe-13,Nle-14]Bn(6-14) has been discovered and the novel pharmacology of this receptor as well as the BB-4-R determined, including their cellular transduction mechanisms. The molecular basis for selectivity of various BN receptor agonists and antagonists is currently being studied. Furthermore, using site-directed mutagenesis the importance of various receptor motifs for participating in receptor modulatory processes (internalization, down-regulation, desensitization) are being explored.